1. Field of the Invention
The present invention relates to a color image processing apparatus for correcting a color in an image output from a printer, a control method for the color image processing apparatus, and a program for creating image processing parameters.
2. Description of the Related Art
In recent years, with the improvement in the performance of electrophotographic devices (Xerographic printers), machines that realize image quality equivalent to that of inkjet printers have been emerging. However, due to their inherent instability, electrophotographic devices have a larger amount of variation in color than inkjet printers. In order to address such a problem, there are various calibration techniques for electrophotographic devices of the related art.
One such calibration technique for electrophotographic devices (xerographic printers) of the related art is used for the correction of primary colors, and involves creating a look-up table (LUT) for use in one-dimensional gradation correction for each of cyan, magenta, yellow, and black toners. A LUT is a table indicating output data corresponding to input data that is divided at specific intervals, and enables the representation of non-linear characteristics, which are difficult to represent with arithmetic expressions. Calibration of a “single color” (hereinafter referred to as “single-color calibration”), which refers to a color represented using a single cyan (C), magenta (M), yellow (Y), or black (K) toner, allows correction of the reproduction characteristics of a single-color image, such as maximum density and gradation.
In recent years, furthermore, a technique for “multi-color” calibration using a four-dimensional LUT has been proposed in Japanese Patent Laid-Open No. 2011-254350. The term “multi-color” refers to a representation of a color produced using a plurality of toners, such as red, green, or blue, which is made up of two of cyan, magenta, and yellow, or gray, which is made up of cyan, magenta, and yellow. Particularly, in the electrophotographic system, even if the gradation characteristics of single colors are corrected using one-dimensional LUTs, a “multi-color” representation produced using a plurality of toners may exhibit a non-linear difference. Multi-color calibration corrects the color reproduction characteristics of a multi-color image represented with a combination (such as superimposition) of a plurality of colors of toners.
The workflow for calibration including multi-color calibration will now be described. First, in order to carry out single-color calibration, a patch image is printed on a recording medium such as a sheet of paper using chart data formed of a single color. The patch image is an image for measurement having a certain area with a single density. A plurality of such patch images are generated with different colors, and are printed on a recording medium. An image formed in this way is referred to as a “pattern image”. A recording medium such as a sheet of paper having a pattern image printed thereon is scanned by a scanner or a sensor to read patch images. One-dimensional LUTs are created in which data obtained by reading the patch images is compared with preset target values to correct the differences from the target values. Then, in order to carry out multi-color calibration, patch images are printed on a recording medium using multi-color chart data that reflects the previously created one-dimensional LUTs, and are read using a scanner or a sensor. A four-dimensional LUT is created in which data obtained by reading the patch images is compared with preset target values to correct the differences from the target values.
Generally, in terms of the reproducibility of colors output from an electrophotographic device (xerographic printer), the influence caused by different image processing methods, which are pseudo-halftone methods, needs to be also taken into account. The image processing methods mainly include two types, e.g., error diffusion and dithering, to implement a pseudo-halftone process. The image processing methods give different gradation characteristics for error diffusion and dithering. Different image processing methods cause the reproducibility of a color to differ. Thus, correction LUTs are used for the respective image processing methods. In other words, calibration is required for every image processing method. However, performing calibration for every image processing method is time and labor consuming. In addition, a large number of consumables such as sheets and toner are also required. To remedy such inconvenience, methods for making single-color calibration as simple as possible have been proposed. Japanese Patent Laid-Open No. 2000-101836 discloses that single-color calibration is performed for one type of image processing method and correction LUTs for the other types of image processing methods are created using conversion tables for image processing methods, which are created by experiment in advance. Thus, LUTs may be created without an increase in user workload.
In the related art, single-color calibration, which would otherwise be performed for every image processing method, is made simple with accuracy maintained as high as possible. The techniques disclosed in the related art thus do not guarantee the reproduction characteristics of a multi-color image.
In general, performing multi-color calibration in a way similar to that for single-color calibration results in an increase in the number of sheets to be used or the amount of toner. Furthermore, multi-color calibration may require consumables such as sheets and toner, and also require a substantial amount of time to perform calibration and user effort as well.